1. Field of the Invention
The present invention relates to an optical system including a radiation source and at least one collector located in the vicinity of the radiation source for collecting the radiation to provide a beam of radiation.
2. Description of the Related Art
Every lithographic projection apparatus utilizes a radiation source. The radiation source provides illumination power. As the radiation source is mostly point like in shape, the radiation emitted by the radiation source is highly divergent in character. Therefore, collectors are used to focus the radiation into a more convergent radiation beam. Radiation that is not collected means a loss in illumination power. Hence, it is important to collect the radiation in as big a solid angle as possible. Multilayer (ML) collectors are able to collect radiation from such a small radiation source in a solid angle of more than 2π sr (although practical angles are in between 1.4π sr to 1.63π sr) and provide a radiation beam. Presently, however, there is no adequate alternative for ML collectors available with a large collector angle.